It is sometimes desirable at an electric utility substation to use more than one load tapchanging (LTC) transformers such as 9, 10, or 11 of FIG. 1a or fixed transformer/regulator combinations such as 39/42, 40/43, or 41/44 shown in FIG. 1b, with the primary inputs and secondary outputs selectively connected in parallel. The term, `tapchanging unit` or `unit` is used hereinunder to be either:
a) a single LTC transformer containing the tapchanging switch and related auxiliary components, or PA1 b) the combination of a fixed ratio transformer directly fed into a tapchanging regulator wherein the combination performs essentially the same task as an LTC transformer.
In either case it is accepted industry practice to provide a central tap switch position with 16 steps of voltage raise and 16 steps of voltage lower on either side of the center. A further industry practice is for each step to give 5/8% output voltage change, thereby providing a maximum range of +/-10% in voltage adjustment.
FIG. 2 shows the tapswitching details of one phase of a three phase tapchanging transformer. At neutral, a buck/boost switch operates and together with a center tapped bridging autotransformer gives a +/-10% voltage regulation range. The output voltage, E, comes from the center tap of the autotransformer with the contacts stepping, for example, from neutral (N) position to a position with one switch contact on (N) and the other on (1), and with the autotransformer dividing the tap to tap voltage of 5/4% down to 5/8% of the output voltage. At the next step, both contacts could be on position (1).
Winding C of transformer T1 is generally a higher voltage than winding B in the normal use of stepping voltages down from transmission level to subtransmission or from sub transmission further down to distribution levels. Alternatively a fixed ratio transformer is used to step the voltage down followed by a regulator for voltage regulation.
FIG. 2 also represents a single phase regulator wherein the voltage is brought in at point A and winding C is not used.
One method of paralleling uses various combinations of auxiliary switches on each transformer in order to sense whether the two tapchanging switches are on the same position and to determine which transformer should move its tapchanger switch next in order to bring the tapchanger switches back to the same tap when a deviation is detected. This method of paralleling often used an auxiliary switch on each tapchanger mechanism that was, say closed on even taps and open on odd taps. This method has fallen into disfavor due to the complexity of the switching circuitry and the auxiliary switch maintenance requirements.
FIG. 3 illustrates the circulating current method of paralleling in most common use today. This illustration is taken from an instruction book by the Beckwith Electric Co. Inc., Largo, Fla. 34643 and illustrates use of the Beckwith Electric Model M-0115 parallel balancing units, Model M-0067 tapchanger controls, Model M-0127 overcurrent relays and Model M-0169 auxiliary current transformers. The network essentially forms a 60 Hz current analog of the quadrature current flowing between the two LTC transformers. Whenever the transformers are not on the same tap positions, the network feeds a measure of the circulating current into the M-0067 controls as a voltage in quadrature with the circulating current in such a way to change and reduce the circulating current towards zero.
This method has the disadvantage of requiring experimental setting of the gain of the system by devices, K3, as seen in the Model M-0115 units of FIG. 3, so as to establish an operating point where the paralleled controls do not hunt and yet operate with tap switches within one or two taps of each other.
A third method is introduced by U.S. Pat. No. 5,210,443 issued to Kurt Kugler on May 11, 1993. The Kugler method uses a parallel digital processor with radial two way data exchange between each tapchanging unit and a central processor. This is essentially a use of the principle of circulating current except that measurements are made in each control sufficient to determine the circulating current but with computations of the circulating currents necessary for paralleling being done in a central computer rather than in each individual control as in the circulating current method, described with reference to FIG. 3.
The prior art also discloses computer programs for solving alternating current (AC) networks wherein elements are phasors. For example, networks as depicted in FIGS. 9 and 10 can be investigated with elements represented either by a series resistive (R) part and a reactive (X) part or by parallel conductance (G) part and a susceptance (B) part. FIGS. 9 and 10 will be discussed in more detail hereinbelow.